CCNA – Semester1

Chapter 4 - OSI Transport Layer

CCNA Exploration version 4.0

Objectives

• Explain the role of Transport Layer protocols and

services in supporting communications across data

networks

• Analyze the application and operation of TCP

mechanisms that support reliability

• Analyze the application and operation of TCP

mechanisms that support reassembly and manage

data loss.

• Analyze the operation of UDP to support

communicate between two processes on end devices

Roles of Transport Layer

Transport Layer Role and Services

• The purpose of the Transport layer:• Enables multiple applications

to communicate over the

network at the same time on a

single device

• Ensures that, if required, all

the data is received reliably

and in order by the correct

application

• Employs error handling

mechanisms

Transport Layer Role and Services

• Major functions of the transport layer and the role it

plays in data networks

• Tracking the individual

communication between

applications on the source and

destination hosts

• Segmenting data and managing

each piece

• Reassembling the segments into

streams of application data

• Identifying the different

applications

Separating Multiple Communications

• Computer connected to a network that is simultaneously receiving and sending e-mail and instant messages, viewing websites, and conducting a VoIP phone call. Each of these applications is sending and receiving data over the network at the same time. However, data from the phone call is not directed to the web browser, and text from an instant message does not appear in an e-mail.

Segmentation

• Without

segmentation, only

one application, the

streaming video for

example, would be

able to receive data.

You could not

receive e-mails, chat

on instant

messenger, or view

web pages while

also viewing the

video.

Controlling the Conversation

Transport layer provides:

– Connection-oriented

conversations

– Reliable delivery

– Ordered data

reconstruction

– Flow control

• Segmentation and Reassembly

• Conversation Multiplexing

Controlling the Conversation

Transport Layer Role and Services

• At the Transport layer the three basic operations of reliability are:

– tracking transmitted data

– acknowledging received data

– retransmitting any unacknowledged data

Transport Layer Role and Services

• Transmission Control Protocol (TCP)

– TCP is a connection-oriented protocol, described in RFC 793. TCP incurs additional overhead to gain functions. Additional functions specified by TCP are the same order delivery, reliable delivery, and flow control. Each TCP segment has 20 bytes of overhead in the header encapsulating the Application layer data, whereas each UDP segment only has 8 bytes of overhead

• UDP is a simple, connectionless protocol, described in RFC 768. It has the advantage of providing for low overhead data delivery: best-effort delivery

Web, email, file transfer

DNS, Video streaming, VoIP

TCP Header format:

TCP Header format: Port number

• Source Port 16 bits.

• Destination Port 16 bits.

TCP Header format: Sequence

• Sequence Number: 32 bits

– The sequence number of the

first data octet in this segment

(except when SYN is present).

TCP Header format: Acknowledgment

• Acknowledgment Number: 32 bits

– This field contains the value of the

next sequence number the sender of

the segment is expecting to receive.

TCP Header format: Code bits

• Control Bits: 8 bits

– ACK: Acknowledgment field significant

– RST: Reset the connection

– SYN: Synchronize sequence numbers

– FIN: No more data from sender

TCP Header format: Window

• Window: 16 bits

– The number of data octets beginning

with the one indicated in the

acknowledgment field which the sender

of this segment is willing to accept.

User Datagram Protocol (UDP)

• UDP is a simple protocol that exchanges datagrams,

without acknowledgments or guaranteed delivery. Error

processing and retransmission must be handled by higher

layer protocols

• UDP is designed for applications that do not need to put

sequences of segments together

UDP header fields

• Source port – Number of the calling port

• Destination port – Number of the called port

• Length – Number of bytes including header and data

• Checksum – Calculated checksum of the header and data fields

• Data – Upper-layer protocol data

• The protocols that use UDP include:

- TFTP (Trivial File Transfer Protocol)

- SNMP (Simple Network Management Protocol)

- DHCP (Dynamic Host Control Protocol)

- DNS (Domain Name System)

Transport Layer Role and Services

• Port number representation and its role

Port and TCP port

• Reserved for services and applications. They are commonly used for applications such as HTTP (web server) POP3/SMTP (e-mail server) and Telnet

• These port numbers are assigned to user processes or applications. When not used for a server resource, these ports may also be used dynamically selected by a client as its source port.

• These are usually assigned dynamically to client applications when initiating a connection. It is not very common for a client to connect to a service using a Dynamic or Private Port

UDP port

TCP and UDP Common Port

TCP and UDP port numbers

• Both TCP and UDP use port (socket) numbers to pass

information to the upper layers. Port numbers are used to

keep track of different conversations crossing the network at

the same time.

Netstat command

Transport Layer Role and Services

• Segmentation and Reassembly.

The TCP Protocol –

Communicating with Reliability

TCP – Making Conversations Reliable

• The key distinction between TCP and UDP is reliability.

TCP Server Processes

• Each application process running on the server is configured to use a port number, either by default or manually by a system administrator.

– Request destination port, request source port

– Response destination port, response source port

3-way handshake

Application and Operation of TCP Mechanisms

Application and Operation of TCP Mechanisms

Lab 4.2.5.2

Managing TCP Sessions

TCP Segment Reassembly

• Resequencing Segments to Order Transmitted

TCP Acknowledgement with Windowing

TCP Retransmission

• Retransmission for lost data employed by TCP

Managing TCP Sessions

Managing TCP Sessions

Managing TCP Sessions

Managing TCP Sessions

TCP Congestion Control – Minimizing

Segment Loss

• Flow Control

TCP Congestion Control – Minimizing

Segment Loss• Reducing window size

Flow Control

When datagrams arrive

too quickly for a host or

gateway to process and

the transport function can

issue a “not ready”

indicator to the sender to

stop sending data. When

the receiver can handle

additional data, the

receiver sends a “ready”

transport indicator. When

this indicator is received,

the sender can resume

the segment transmission.

The UDP Protocol –

Communicating with Low overhead

UDP – Low overhead vs. Reliability

• UDP is a simple protocol that provides the basic Transport layer functions. It much lower overhead than TCP, since it is not connection-oriented and does not provide the sophisticated retransmission, sequencing, and flow control mechanisms.

– Domain Name System (DNS)

– Simple Network Management Protocol (SNMP)

– Dynamic Host Configuration Protocol (DHCP)

– Routing Information Protocol (RIP)

– Trivial File Transfer Protocol (TFTP)

– Online games

UDP Datagram Reassembly

• UDP simply reassembles the data in the order that it was

received and forwards it to the application.

UDP Server Processes and Requests

• Like TCP-based applications, UDP-based server

applications are assigned Well Known or Registered port

numbers.

UDP Client Processes

• As with TCP, client/server communication is initiated by a

client application that is requesting data from a server

process: using ports

Lab 4.2.5.2

Summary